The use of thick film conductors in hybrid microelectronic components is well known in the electronic field. Such materials are usually comprised of a dispersion of finely divided particles of a noble metal, noble metal alloy or mixtures thereof and a minor amount of inorganic binder, both dispersed in an organic medium to form a pastelike product. The consistency and rheology of the paste is adjusted to the particular method of application such as screen printing, brushing, dipping, extrusion, spraying and the like. Such pastes are usually applied to an inert substrate such as Al.sub.2 O.sub.3 by screen printing to form a patterned layer. The patterned thick film conductor layer is then fired to volatilize the organic medium and sinter the inorganic binder, which is usually glass or a glass-forming material.
In addition to the electrical conductivity properties which the fired conductive layer must possess, it is essential that it adhere firmly to the substrate on which it is printed and that the layer be capable of accepting solder. Solderability is, of course, essential because of the necessity of connecting the conductive pattern with other components of the electronic system in which it is used, e.g., resistor and capacitor networks, resistors, trim potentiometers, chip resistors, chip capacitors, chip carriers and the like.
Most firing of thick film materials has been carried out in belt furnaces which utilize as a heat source heavy gauge resistance-wire elements embedded in a refractory surrounding a quartz or metal muffle. Because the mass of such furnaces is high the response of the furnaces is slow and they operate at temperatures close to the temperature of the products being fired therein. Because of this small temperature difference, the rate at which energy is delivered to the product is restricted. Until recently such muffle furnaces were as a practical matter the only viable choice for firing thick films. Using conventional muffle furnaces the firing of thick films is usually carried out using firing profiles of 30-60 minutes duration in which the peak temperature is typically 850.degree. C., heating rate is 35.degree.-100.degree. C./minute, peak dwell time is 5-10 minutes and the cooling rate is 35.degree.-150.degree. C./minute. To provide for safe removal of organic media without blistering, chemical reactions or mechanical strain it was necessary to use relatively slow heat-up rates, long dwell times and controlled slow cooling.
In response, however, to the economic desirability and competitive need to improve productivity by most manufacturers of electronic components who utilize thick film technology, there exists a strong trend toward the use of infrared belt furnaces. Infrared furnaces utilize as energy source a filament of tungsten or nickel-chromium alloy which is heated electrically to a temperature substantially above the chamber or product temperatures. Lamps incorporating these filaments are mounted in arrays above and below the belt. Because of the large temperature differences between the filaments and the parts being fired the furnace can deliver very large amount of energy to the part being fired much more rapidly than in conventional muffle furnaces. Thus, parts that require firing profiles of 20 minutes to peak temperature, 10 minutes at peak temperature and 15 minutes down to 100.degree. C. in conventional belt furnaces can now be fired effectively with profiles of 81/2 minutes to peak temperature and 61/2 minutes down to 100.degree. C.
As might be expected from the complexity of the many reactions and interactions which take place during firing, it has been found that many materials which are imminently suitable for conventional slow firing are less suitable for more rapid firing in infrared equipment.
Thus, while extensive research has been directed to the problems of thick film conductor adhesion and solderability, it has almost entirely been directed to the problems associated with conventional slow firing furnaces and very little has been directed to fast firing in infrared furnaces.